1. Field of the Invention
The invention is directed to an improved fuel injection valve for internal combustion engines.
2. Description of the Prior Art
International Patent Disclosure WO 96/19661 shows a fuel injection valve with a valve body, in which a bore is embodied that is defined on its end toward the combustion chamber by a conical valve seat. A pistonlike valve needle is disposed longitudinally displaceably in the bore and has an essentially conical valve sealing face on its end toward the combustion chamber. The valve sealing face is separated into two conical faces, which are separated from one another by an annular groove. The opening angle of the two conical faces and the opening of the conical valve seat are adapted to one another in such a way that upon contact of the valve needle with the valve seat, the edge that is embodied at the transition from the annular groove to the first conical face comes to rest on the valve seat and acts as a sealing edge, in order to control the flow of fuel to at least one injection opening that originates at the valve seat and discharges into the combustion chamber of the engine.
The second edge of the annular groove, which along with the sealing edge defines the annular groove and is embodied at the transition to the second conical face at the valve sealing face, is spaced apart from the valve seat in the closing position of the valve needle, or in other words when the valve needle comes to rest with its sealing edge on the valve seat. The valve needle is kept in its closing position by a closing force because a closing force that presses the valve needle against the valve seat acts on its end facing away from the combustion chamber. In order for the valve needle to uncover the injection openings, a hydraulic contrary force that exceeds the closing force must act on the valve needle. Given a suitable pressure in the pressure chamber that is embodied between the valve needle and the wall of the bore, the result is a corresponding hydraulic force exerted, among other places, on parts of the valve sealing face, resulting in a corresponding opening force oriented counter to the closing force. If the valve needle then lifts from the valve seat, fuel flows out of the pressure chamber to the injection openings between the valve seat and the valve sealing face.
In the partial stroke range, that is, before the valve needle has reached its maximum opening stroke, the problem arises that because of the inflowing fuel that prevails in the pressure chamber at high pressure, the pressure in the annular groove also rises. A further flow to the injection openings is initially possible only in throttled fashion, since the gap between the second edge of the annular groove and the valve seat assures a corresponding throttling effect, especially whenever, over the course of usage, the spacing between the second edge and the valve seat becomes increasingly less because of wear, or even vanishes entirely in the closing position of the valve needle. This elevated pressure in the annular groove causes an additional opening force on the valve needle that is not initially present and that changes the opening speed and hence also the instant at which the valve needle reaches its maximum opening. Thus over time, the opening dynamics of the valve needle and hence the fuel quantity injected vary. For precise fuel injection of the kind necessary in high-speed, self-igniting internal combustion engines, this change in the opening dynamics means that optimal injection in terms of pollutant emissions and fuel consumption is no longer assured.